Electron beam machining (EBM)

WORKING PRINCIPLE OF EBM AND ITS MACHINING APPLICATIONS

1. Electron Beam Machining (EBM) BY, ARAVINDKUMAR B

2. Electron Beam Machining – Process Electron beam is generated in an electron beam gun. Electron beam gun provides high velocity electrons over a very small spot size. Electron Beam Machining is required to be carried out in vacuum. Otherwise the electrons would interact with the air molecules, thus they would loose their energy and cutting ability. Thus the workpiece to be machined is located under the electron beam and is kept under vacuum.

3. The high-energy focused electron beam is made to impinge on the workpiece with a spot size of 10 – 100 μm . The kinetic energy of the high velocity electrons is converted to heat energy as the electrons strike the work material. Due to high power density instant melting and vaporisation starts and “melt – vaporisation ” front gradually progresses Finally the molten material, if any at the top of the front, is expelled from the cutting zone by the high vapour pressure at the lower part Holes can be drilled in thin sheets using a single pulse. For thicker plates, multiple pulses would be required.

4. Electron Beam Machining – Equipment The basic functions of any electron beam gun are to generate free electrons at the cathode, accelerate them to a sufficiently high velocity and to focus them over a small spot size. Cathode catridge made of tungsten or tantalum. Such cathode filaments are heated, to a temperature of around 2500 deg C Such heating leads to thermo-ionic emission of electrons, which is further enhanced by maintaining very low vacuum within the chamber of the electron beam gun

5. Moreover, this cathode cartridge is highly negatively biased so that the thermo-ionic electrons are strongly repelled away form the cathode. A high negative bias is applied to this grid so that the electrons generated by this cathode do not diverge and approach the next element, the annular anode, in the form of a beam. The annular anode now attracts the electron beam and gradually gets accelerated. As they leave the anode section, the electrons may achieve a velocity as high as half the velocity of light. The magnetic lenses shape the beam and try to reduce the divergence. Apertures on the other hand allow only the convergent electrons to pass and capture the divergent low energy electrons from the fringes.

6. The electromagnetic lens focuses the electron beam to a desired spot. The deflection coil can manoeuvre the electron beam, though by small amount, to improve shape of the machined holes. Generally in between the electron beam gun and the workpiece , which is also under vacuum, there would be a series of slotted rotating discs. Such discs allow the electron beam to pass and machine materials but helpfully prevent metal fumes and vapour generated during machining to reach the gun.

7. advandages EBM provides very high drilling rates when small holes with large aspect ratio are to be drilled. As it applies no mechanical cutting force, work holding and fixturing cost is very less. Further for the same reason fragile and brittle materials can also be processed. The heat affected zone in EBM is rather less due to shorter pulses. EBM can provide holes of any shape by combining beam deflection using electromagnetic coils and the CNC table with high accuracy.

8. Dis advandages The primary limitations are the high capital cost of the equipment and necessary regular maintenance applicable for any equipment using vacuum system. Moreover in EBM there is significant amount of non-productive pump down period for attaining desired vacuum.